Dose perturbation induced by radiographic contrast inside brachytherapy balloon applicators

Phantom measurements and Monte Carlo calculations have been performed for the purpose of characterizing the dose perturbation caused by radiographic contrast inside the MammoSite breast brachytherapy applicator. Specifically, the dose perturbation is quantified as a heterogeneity correction factor (HCF) for various balloon radii and contrast concentration levels. The dose perturbation is larger for larger balloon radii and higher contrast concentrations. Based on a validated Monte Carlo simulation, the calculated HCF values are 0.99 for a 2 cm radius balloon and 0.98 for a 3 cm radius balloon at 6% contrast concentration levels, and 0.89 and 0.87 for 2 and 3 cm radius balloons, respectively, at 100% contrast concentrations. For a typical implanted balloon radius of 2.4 cm, the HCF values decrease from 0.99 at 6% contrast concentration to 0.90 at 100% contrast concentration. For balloons implanted in patients at our institution, the mean HCF is 0.99, corresponding to a dose reduction of approximately 1%. The contrast effect results in a systematic reduction in the delivered dose, therefore the minimal amount of radiographic contrast necessary should be used.     

Monte carlo simulation of the Leksell Gamma Knife: II. Effects of heterogeneous versus homogeneous media for stereotactic radiosurgery

The absence of electronic equilibrium in the vicinity of bone-tissue or air-tissue heterogeneity in the head can misrepresent deposited dose with treatment planning algorithms that assume all treatment volume as homogeneous media. In this paper, Monte Carlo simulation (PENELOPE) and measurements with a specially designed heterogeneous phantom were applied to investigate the effect of air-tissue and bone-tissue heterogeneity on dose perturbation with the Leksell Gamma Knife. The dose fall-off near the air-tissue interface caused by secondary electron disequilibrium leads to overestimation of dose by the vendor supplied treatment planning software (GammaPlan) at up to 4 mm from an interface. The dose delivered to the target area away from an air-tissue interface may be underestimated by up to 7% by GammaPlan due to overestimation of attenuation of photon beams passing through air cavities. While the underdosing near the air-tissue interface cannot be eliminated with any plug pattern, the overdosage due to under-attenuation of the photon beams in air cavities can be eliminated by plugging the sources whose beams intersect the air cavity. Little perturbation was observed next to bone-tissue interfaces. Monte Carlo results were confirmed by measurements. This study shows that the employed Monte Carlo treatment planning is more accurate for precise dosimetry of stereotactic radiosurgery with the Leksell Gamma Knife for targets in the vicinity of air-filled cavities.     

On-Board Imager-based MammoSite treatment verification

Contemporary radiation oncology departments are often lacking a conventional simulator due to common use of virtual simulation and recent implementation of image guided radiation therapy. A protocol based on MammoSite method was developed using CT based planning, a Source Position Simulator (SPS) with a Simulator Wire and a linear accelerator based On-Board Imager (OBI) for daily verification. After MammoSite balloon implantation, the patient undergoes a CT study. The images are evaluated for tissue conformance, balloon symmetry, and balloon surface to skin distance according to the departmental procedure. Prior to the CT study the SPS is attached to the transfer tube that in turn is attached to the balloon catheter. The length from the indexer to the first dwell position is measured using the simulator wire with X-ray markers. After the CT study is performed, the data set is sent to the Varian Eclipse treatment planning system (TPS) and to the Nucletron PLATO brachytherapy planning system. The reference digitally reconstructed radiographs (DRRs) of anterior and lateral setup fields are created using Eclipse TPS and are immediately available on the OBI console via the Varian Vision integrated system. The source dwell position coinciding with the balloon center is identified in the CT dataset, followed by the offset calculation, catheter reconstruction, dose points placement and dwell time calculation. OBI fluoroscopy images are acquired and marked as initial. Prior to each treatment fraction balloon diameter and symmetry are evaluated using OBI fluoroscopy and tools available on the OBI console. Acquired images are compared with reference DRRs and/or initial OBI images. The whole process from initial evaluation to daily verification is filmless and does not undermine the precision of the procedure. This verification time does not exceed 10 min. The balloon diameter correlates well (within 1 mm) between initial CT and OBI verification images. The balloon symmetry is defined with 1 mm accuracy using existing OBI console tools. It is feasible to use OBI based simulation for the MammoSite balloon placement evaluation, balloon integrity daily verification, and treatment dwell position coincidence with balloon center. This verification is a rapid process and is an alternative to the conventional simulator based technique. The simulator wire with X-ray markers for the SPS is the recommended tool for the CT based MammoSite procedure.     

Experimental validation of dose calculation algorithms for the GliaSite RTS, a novel 125I liquid-filled balloon brachytherapy applicator

This paper compares experimentally measured and calculated dose-rate distributions for a novel 125I liquid-filled brachytherapy balloon applicator (the GliaSite RTS), designed for the treatment of malignant brain-tumor resection-cavity margins. This work is intended to comply with the American Association of Physicists in Medicine (AAPM) Radiation Therapy Committee's recommendations [Med. Phys. 25, 2269-2270 (1998)] for dosimetric characterization of low-energy photon interstitial brachytherapy sources. Absolute low dose-rate radiochromic film (RCF) dosimetry measurements were performed in coronal planes about the applicator. The applicator was placed in a solid water phantom, machined to conform to the inflated applicator's surface. The results were used to validate the accuracy of Monte Carlo photon transport (MCPT) simulations and a point-source dose-kernel algorithm in predicting dose to water. The absolute activity of the 125I solution was determined by intercomparing a National Institute of Standards and Technology (NIST) 125I standard with a known mass of radiotherapy solution (Iotrex) in an identical vial and geometry. For the two films not in contact with applicator, the average agreement between RCF and MCPT (specified as the mean absolute deviation in successive 4 mm rings) was found to be within +/-5% at distances 0.2-25 mm from the film centers. For the two films touching the catheter, the mean agreement was +/-14.5% and 7.5% near the balloon surface but improving to 7.5% and 6% by 3.5 mm from the surface. These errors, as large as 20% in isolated pixels, are likely due to trim damage, 125I contamination, and poor conformance with the balloon. At larger distances where the radiation doses were very low, the observed discrepancies were significantly larger than expected. We hypothesize that they are due to a dose-rate dependence of the RCF response. A 1%-10% average difference between a simple one-dimensional path-length semiempirical dose-kernel model and the MCPT calculations was observed over clinically relevant distances.     

The dose distribution of low dose rate Cs-137 in intracavitary brachytherapy: comparison of Monte Carlo simulation, treatment planning calculation and polymer gel measurement

In this study, the dose distribution delivered by low dose rate Cs-137 brachytherapy sources was investigated using Monte Carlo (MC) techniques and polymer gel dosimetry. The results obtained were compared with a commercial treatment planning system (TPS). The 20 mm and the 30 mm diameter Selectron vaginal applicator set (Nucletron) were used for this study. A homogeneous and a heterogeneous-with an air cavity-polymer gel phantom was used to measure the dose distribution from these sources. The same geometrical set-up was used for the MC calculations. Beyond the applicator tip, differences in dose as large as 20% were found between the MC and TPS. This is attributed to the presence of stainless steel in the applicator and source set, which are not considered by the TPS calculations. Beyond the air cavity, differences in dose of around 5% were noted, due to the TPS assuming a homogeneous water medium. The polymer gel results were in good agreement with the MC calculations for all the cases investigated.     

A dosimetric comparison of 169Yb and 192Ir for HDR brachytherapy of the breast, accounting for the effect of finite patient dimensions and tissue inhomogeneities

Monte Carlo simulation dosimetry is used to compare 169Yb to 192Ir for breast high dose rate (HDR) brachytherapy applications using multiple catheter implants. Results for bare point sources show that while 169Yb delivers a greater dose rate per unit air kerma strength at the radial distance range of interest to brachytherapy in homogeneous water phantoms, it suffers a greater dose rate deficit in missing scatter conditions relative to 192Ir. As a result of these two opposing factors, in the scatter conditions defined by the presence of the lung and the finite patient dimensions in breast brachytherapy the dose distributions calculated in a patient equivalent mathematical phantom by Monte Carlo simulations for the same implant of either 169Yb or 1921r commercially available sources are found comparable. Dose volume histogram results support that 169Yb could be at least as effective as 192Ir delivering the same dose to the lung and slightly reduced dose to the breast skin. The current treatment planning systems' approach of employing dosimetry data precalculated in a homogeneous water phantom of given shape and dimensions, however, is shown to notably overestimate the delivered dose distribution for 169Yb. Especially at the skin and the lung, the treatment planning system dose overestimation is on the order of 15%-30%. These findings do not undermine the potential of 169Yb HDR sources for breast brachytherapy relative to the most commonly used 192Ir HDR sources. They imply, however, that there could be a need for the amendment of dose calculation algorithms employed in clinical treatment planning of particular brachytherapy applications, especially for intermediate photon energy sources such as 169Yb.     

The effect of patient inhomogeneities in oesophageal 192Ir HDR brachytherapy: a Monte Carlo and analytical dosimetry study

The effect of patient inhomogeneities surrounding the oesophagus on the dosimetry planning of an upper thoracic oesophageal 192Ir HDR brachytherapy treatment is studied. The MCNPX Monte Carlo code is used for dosimetry in a patient-equivalent phantom geometry and results are compared in terms of isodose contours as well as dose volume histograms with corresponding calculations by a contemporary treatment planning system software featuring a full TG-43 dose calculation algorithm (PLATO BPS version 14.2.4). It is found that the presence of patient inhomogeneities does not alter the delivery of the planned dose distribution to the planning treatment volume. Regarding the organs at risk, the common practice of current treatment planning systems (TPSs) to consider the patient geometry as a homogeneous water medium leads to a dose overestimation of up to 13% to the spinal cord and an underestimation of up to 15% to the sternum bone. These findings which correspond to the dose region of about 5-10% of the prescribed dose could only be of significance when brachytherapy is used as a boost to external beam therapy. Additionally, an analytical dosimetry model, which is efficient in calculating dose in mathematical phantoms containing inhomogeneity shells of materials of radiobiological interest, is utilized for dosimetry in the patient-equivalent inhomogeneous phantom geometry. Analytical calculations in this work are in good agreement with corresponding Monte Carlo results within the bone inhomogeneities of spinal cord and sternum bone but, like treatment planning system calculations, the model fails to predict the dose distribution in the proximal lung surface as well as within the lungs just as the TPS does, due to its inherent limitation in treating lateral scatter and backscatter radiation.     

Clinical implementation of full Monte Carlo dose calculation in proton beam therapy

The goal of this work was to facilitate the clinical use of Monte Carlo proton dose calculation to support routine treatment planning and delivery. The Monte Carlo code Geant4 was used to simulate the treatment head setup, including a time-dependent simulation of modulator wheels (for broad beam modulation) and magnetic field settings (for beam scanning). Any patient-field-specific setup can be modeled according to the treatment control system of the facility. The code was benchmarked against phantom measurements. Using a simulation of the ionization chamber reading in the treatment head allows the Monte Carlo dose to be specified in absolute units (Gy per ionization chamber reading). Next, the capability of reading CT data information was implemented into the Monte Carlo code to model patient anatomy. To allow time-efficient dose calculation, the standard Geant4 tracking algorithm was modified. Finally, a software link of the Monte Carlo dose engine to the patient database and the commercial planning system was established to allow data exchange, thus completing the implementation of the proton Monte Carlo dose calculation engine ('DoC++'). Monte Carlo re-calculated plans are a valuable tool to revisit decisions in the planning process. Identification of clinically significant differences between Monte Carlo and pencil-beam-based dose calculations may also drive improvements of current pencil-beam methods. As an example, four patients (29 fields in total) with tumors in the head and neck regions were analyzed. Differences between the pencil-beam algorithm and Monte Carlo were identified in particular near the end of range, both due to dose degradation and overall differences in range prediction due to bony anatomy in the beam path. Further, the Monte Carlo reports dose-to-tissue as compared to dose-to-water by the planning system. Our implementation is tailored to a specific Monte Carlo code and the treatment planning system XiO (Computerized Medical Systems Inc.). However, this work describes the general challenges and considerations when implementing proton Monte Carlo dose calculation in a clinical environment. The presented solutions can be easily adopted for other planning systems or other Monte Carlo codes.     

A systematic evaluation of air cavity dose perturbation in megavoltage x-ray beams

The EGS4 Monte Carlo radiation transport code was used to systematically study the dose perturbation near planar and cylindrical air cavities in a water medium irradiated by megavoltage x-ray beams. The variables of the problem included x-ray energy, cavity shape and dimension, and depth of the cavity in water. The Monte Carlo code was initially validated against published measurements and its results were found to agree within 2% with the published measurements. The study results indicate that the dose perturbation is strongly dependent on x-ray energy, field size, depth, and size of cavity in water. For example, the Monte Carlo calculations show dose reductions of 42% and 18% at 0.05 and 2 mm, respectively, beyond the air-water interface distal to the radiation source for a 3 cm thick air slab irradiated by a single 5x5 cm2 15 MV beam. The dose reductions are smaller for a parallel-opposed pair of 5x5 cm2 15 MV x-ray beams, being 21% and 11% for the same depths. The combined set of Monte Carlo calculations showed that the dose reduction near an air cavity is greater for: (a) Smaller x-ray field size, (b) higher x-ray energy, (c) larger air-cavity size, and (d) smaller depth in water where the air cavity is situated. A potential clinical application of these results to the treatment of prostate cancer is discussed.     

Accurate localization of intracavitary brachytherapy applicators from 3D CT imaging studies

PURPOSE: To present an accurate method to identify the positions and orientations of intracavitary (ICT) brachytherapy applicators imaged in 3D CT scans, in support of Monte Carlo photon-transport simulations, enabling accurate dose modeling in the presence of applicator shielding and interapplicator attenuation. MATERIALS AND METHODS: The method consists of finding the transformation that maximizes the coincidence between the known 3D shapes of each applicator component (colpostats and tandem) with the volume defined by contours of the corresponding surface on each CT slice. We use this technique to localize Fletcher-Suit CT-compatible applicators for three cervix cancer patients using post-implant CT examinations (3 mm slice thickness and separation). Dose distributions in 1-to-1 registration with the underlying CT anatomy are derived from 3D Monte Carlo photon-transport simulations incorporating each applicator's internal geometry (source encapsulation, high-density shields, and applicator body) oriented in relation to the dose matrix according to the measured localization transformations. The precision and accuracy of our localization method are assessed using CT scans, in which the positions and orientations of dense rods and spheres (in a precision-machined phantom) were measured at various orientations relative to the gantry. RESULTS: Using this method, we register 3D Monte Carlo dose calculations directly onto post insertion patient CT studies. Using CT studies of a precisely machined phantom, the absolute accuracy of the method was found to be +/-0.2 mm in plane, and +/-0.3 mm in the axial direction while its precision was +/-0.2 mm in plane, and +/-0.2 mm axially. CONCLUSION: We have developed a novel, and accurate technique to localize intracavitary brachytherapy applicators in 3D CT imaging studies, which supports 3D dose planning involving detailed 3D Monte Carlo dose calculations, modeling source positions, shielding and interapplicator shielding, accurately.     

Dose perturbation effects in prostate seed implant brachytherapy with I-125

EGSnrc Monte Carlo simulation was used to investigate dose perturbation effects in prostate seed implant brachytherapy using 125I radioactive seeds used in implant brachytherapy. Dose perturbation effects resulting from the seed mutual attenuation in a prostate seed implant consisting of 27 seeds were investigated. The results showed that for 125I seeds implanted into the prostate at 1.00 cm, 0.75 cm and 0.50 cm apart (uniform spacing), the dose perturbation effects are up to 10%. The volume of the target occupied by the 10% dose difference between the full Monte Carlo simulation and the single seed superposition model decreases with increasing seed spacing. Despite the differences between the Monte Carlo simulation and the simple superposition, there was no significant change in the dose volume histogram for 1 cm and 0.75 cm seed spacing. However, there was a significant change in the dose volume histogram when the seed spacing was 0.5 cm. An analysis of the external volume index (EI), coverage index (CI) and homogeneity index (HI) also showed that there is no difference in these indexes for the 1.00 cm and 0.75 cm seed spacing between the simple superposition model and the full Monte Carlo simulation. Compared to the full Monte Carlo simulations, the simple superposition model overestimated EI, CI and HI by 7%, 5% and 4% respectively for the 0.50 cm seed spacing.     

Dose distribution for endovascular brachytherapy using Ir-192 sources: comparison of Monte Carlo calculations with radiochromic film measurements

An analysis of Ir-192 source distribution using the Monte Carlo method and radiochromic film experiments for endovascular brachytherapy is presented. Three different source possibilities, namely, mHDR Ir-192 sources with 5 mm and 2.5 mm step sizes and Ir-192 seed sources with 1 mm air gap are investigated to obtain uniform radial dose distribution throughout the treatment area. From this study, it is inferred that mHDR Ir-192 sources with 2.5 mm step size are effective for getting dose uniformity. Hence, different restenosis geometries, namely, linear, dumb bell and hairpin, are simulated with 2.5 mm step size, 15 mHDR Ir-192 sources using the Monte Carlo technique and the results are compared experimentally by using radiochromic films. The results from both methods agreed to within 7%. Further, it is also inferred that for the dosimetry of endovascular brachytherapy, the film dosimetry may be considered adequate, even if the film calibration is time consuming and requires adequate dosimetric procedures.     

Monte Carlo dose characterization of a new 90Sr/90Y source with balloon for intravascular brachytherapy

Beta emitting source wires or seeds have been adopted in clinical practice of intravascular brachytherapy for coronary vessels. Due to the limitation of penetration depth, this type of source is normally not applicable to treat vessels with large diameter, e.g., peripheral vessel. In the effort to extend application of its beta source for peripheral vessels, Novoste has recently developed a new catheter-based system, the Corona 90Sr/90Y system. It is a source train of 6 cm length and is jacketed by a balloon. The existence of the balloon increases the penetration of the beta particles and maintains the source within a location away from the vessel wall. Using the EGSnrc Monte Carlo system, we have calculated the two-dimensional (2-D) dose rate distribution of the Corona system in water for a balloon diameter of 5 mm. The dose rates on the transverse axis obtained in this study are in good agreement with calibration results of the National Institute of Standards and Technology for the same system for balloon diameters of 5 and 8 mm. Features of the 2-D dose field were studied in detail. The dose parameters based on AAPM TG-60 protocol were derived. For a balloon diameter of 5 mm, the dose rate at the reference point (defined as r0 = 4.5 mm, 2 mm from the balloon surface) is found to be 0.01028 Gy min(-1) mCi(-1). A new formalism for a better characterization of this long source is presented. Calculations were also performed for other balloon diameters. The dosimetry for this source is compared with a 192Ir source, commonly used for peripheral arteries. In conclusion, we have performed a detailed dosimetric characterization for a new beta source for peripheral vessels. Our study shows that, from dosimetric point of view, the Corona system can be used for the treatment of an artery with a large diameter, e.g., peripheral vessel.     

Dosimetry of 192Ir sources used for endovascular brachytherapy

An in-phantom calibration technique for 192Ir sources used for endovascular brachytherapy is presented. Three different source lengths were investigated. The calibration was performed in a solid phantom using a Farmer-type ionization chamber at source to detector distances ranging from 1 cm to 5 cm. The dosimetry protocol for medium-energy x-rays extended with a volume-averaging correction factor was used to convert the chamber reading to dose to water. The air kerma strength of the sources was determined as well. EGS4 Monte Carlo calculations were performed to determine the depth dose distribution at distances ranging from 0.6 mm to 10 cm from the source centre. In this way we were able to convert the absolute dose rate at 1 cm distance to the reference point chosen at 2 mm distance. The Monte Carlo results were confirmed by radiochromic film measurements, performed with a double-exposure technique. The dwell times to deliver a dose of 14 Gy at the reference point were determined and compared with results given by the source supplier (CORDIS). They determined the dwell times from a Sievert integration technique based on the source activity. The results from both methods agreed to within 2% for the 12 sources that were evaluated. A Visual Basic routine that superimposes dose distributions, based on the Monte Carlo calculations and the in-phantom calibration, onto intravascular ultrasound images is presented. This routine can be used as an online treatment planning program.     

Dose rate table for a 32P intravascular brachytherapy source from Monte Carlo calculations

Studies of intravascular brachytherapy to prevent restenosis following angioplasty have shown many promising results. Accurate dose rate tables based on detailed models of the brachytherapy sources are necessary for treatment planning. This work will present an away and along dose rate table for a 27 mm long catheter based 32P beta source. MD-55-2 radiochromic film has been exposed at five different depths (0.5 mm-4 mm) in a polystyrene phantom using a 27 mm long Guidant 32P beta source. The total dose to the active region of the film was determined using the absolute detector response of the MD-55-2 radiochromic film. The Monte Carlo code MCNP4B2 was also used to calculate the dose to the active region of the film using a detailed model of the source, encapsulation, and radiochromic film. The dose to film calculations showed good agreement with the measurements presented in this work with an average difference of 7%. The Monte Carlo calculations were also verified against previously published depth dose in water measurements determined using radiochromic film and plastic scintillator. The depth dose calculations in water showed good agreement with the previously published measurements with the calculations being about 2.5% lower than the film measurements and about 2.5% higher than the scintillator measurements. This work then uses the verified Monte Carlo code to present a dose rate table for the 32P intravascular beta source.     

Use of Monte Carlo computation in benchmarking radiotherapy treatment planning system algorithms

Radiotherapy treatments are becoming more complex, often requiring the dose to be calculated in three dimensions and sometimes involving the application of non-coplanar beams. The ability of treatment planning systems to accurately calculate dose under a range of these and other irradiation conditions requires evaluation. Practical assessment of such arrangements can be problematical, especially when a heterogeneous medium is used. This work describes the use of Monte Carlo computation as a benchmarking tool to assess the dose distribution of external photon beam plans obtained in a simple heterogeneous phantom by several commercially available 3D and 2D treatment planning system algorithms. For comparison, practical measurements were undertaken using film dosimetry. The dose distributions were calculated for a variety of irradiation conditions designed to show the effects of surface obliquity, inhomogeneities and missing tissue above tangential beams. The results show maximum dose differences of 47% between some planning algorithms and film at a point 1 mm below a tangentially irradiated surface. Overall, the dose distribution obtained from film was most faithfully reproduced by the Monte Carlo N-Particle results illustrating the potential of Monte Carlo computation in evaluating treatment planning system algorithms.